UC-NRLF 


QC 


LIBRARY 


UNIVERSITY  OF  CALIFORNIA. 


RECEIVED    BY   EXCHANGE 


Class 


An   Investigation   of  Manometers,  of 
Small  Bore,  for  Use  in  the  Measure- 
ment of  Osmotic  Pressure 


DISSERTATION 


SUBMITTED   TO  THE   BOARD   OF  UNIVERSITY  STUDIES   OF 

JOHNS  HOPKINS  UNIVERSITY  IN  CONFORMITY  WITH 

A  REQUIREMENT  FOR  THE  DEGREE  OF  DOCTOR 

OF  PHILOSOPHY. 


BY 


JOHN  LATTIMORE  CARPENTER 


BALTIMORE 
1911 


EASTON,  PA.: 

ESCHENBACH  PRINTING  COMPANY. 
1911. 


An   Investigation   of  Manometers,  of 
Small  Bore,  for  Use  in  the  Measure- 
ment of  Osmotic  Pressure 


DISSERTATION 


SUBMITTED   TO   THE   BOARD   OF  UNIVERSITY   STUDIES   OF 

JOHNS  HOPKINS  UNIVERSITY  IN  CONFORMITY  WITH 

A  REQUIREMENT  FOR  THE  DEGREE  OF  DOCTOR 

OF  PHILOSOPHY. 


BY 


JOHN  LATTIMORE  CARPENTER 


BALTIMORE 
1911 


EASTON,  PA.: 

ESCHBNBACH  PRINTING  COMPANY. 
1911. 


C3 


CONTENTS. 


Acknowledgment 4 

Review  of  Former  Work 5 

A  New  Type  of  Manometer  and  Discussion  of  the  Same 7 

Determination  of  Capillary  Depression: 

(a)  Purification  of  Mercury 10 

(6)  Discussion  of  Capillary  Depression 1 1 

Filling  the  Manometers : 

(a)  Preparation  of  the  Nitrogen 15 

(6)  Filling  and  Closing  the  Manometer 16 

Determination  of  Gas  Volumes 16 

Comparison  of  Manometers 25 

Summary 29 

Biography 30 


222274 


ACKNOWLEDGMENT. 

The  writer  takes  occasion  here  to  express  his  appreciation 
to  Pres.  Remsen,  Prof.  Morse,  Prof.  Renouf,  Prof.  Jones, 
Prof.  Acree  and  Dr.  C.  K.  Swartz  for  instruction  and  in- 
spiration. 

Especial  thanks  are  due  Prof.  Morse  and  Prof.  Renojf. 
The  former  for  guidance  and  suggestion  through  three  years 
work,  the  latter  for  many  instructive  conversations  upon 
general  chemistry. 

Dr.  Holland  and  Mr.  Cash  are  due  the  writers  thanks  for 
their  willing  assistance  from  time  to  time. 


AN   INVESTIGATION   OF   MANOMETERS   OF  SMALL 

BORE  FOR  USE  IN  THE  MEASUREMENT  OF 

OSMOTIC  PRESSURE. 


Review  of  former  work.1 

Throughout  the  ten  or  more  years  of  investigation  of 
osmotic  pressure  in  this  laboratory  a  major  portion  of  the 
time  has  been  spent  in  the  detection  and  elimination  of 
sources  of  error.  In  order  that  the  problem  might  be  at- 
tacked upon  a  secure  basis,  the  following  factors  must  become 
known  and  reliable  quantities,  viz.:  a  suitably  strong  semi- 
permeable  membrane;  an  effective  method  for  depositing 
the  same;  a  cell  of  fine  texture,  suitable  porosity  and  at  the 
same  time  possessing  great  strength  of  wall ;  baths  accurately 
and  automatically  regulated  as  to  temperature;  and  finally, 
manometers  of  a  type  convenient  to  manipulate  for  registra- 
tion of  the  pressure  exerted  within  the  cell.  Examination 
of  the  published  work  will  reveal  how  elaborate  and  perfect 
the  system  has  come  to  be.  In  fact  all  the  larger  difficulties 
have  been  disposed  of  rather  satisfactorily  except  the  manom- 
eter factor. 

Sufficiently  accurate  manometers  for  the  measurement 
of  the  temperature  coefficient  of  osmotic  pressure  have  been 
in  use  for  three  years  and  their  preparation  has  been  described.2 
But  it  was  realized  that  these  instruments  had  constant 
error  factors — though  of  uncertain  magnitude — and  it  was 
with  difficulty  that  an  adequate  number  of  the  instruments 
agreeing  sufficiently  well  among  themselves  were  finally 
obtained.  By  the  methods  of  manometer  preparation 
formerly  used  one  was  by  no  means  certain  whether  he  would 
obtain  a  "good"  instrument.  The  worker  was  constantly 

1  Earlier  papers  will  be  found  in  Am.  Ch.  Jr.,  28,  i;  29,  173;  32,  93; 
34,  i;  36,  i  and  39;  37,  324;  425,  558;  38,  175;  39,  667;  40,  i,  194,  266, 
325;  41,  i,  92,  257;  45,  91. 

2  Am.  Chem.  Jour.,  40,  325. 


fearing  the  worst  and  in  the  majority  of  cases  his  fears  were 
realized.  However,  a  series  of  manometers  came  into  hand 
which  agreed  closely,  and  these  were  chosen  for  the  measure- 
ments. 

These  manometers  have  been  prepared  substantially,  as 
follows:  the  capillary1  tubes  were  chosen  with  care,  extra 
efforts  being  made  to  find  those  of  most  uniform  bore;  the 
tubes  were  then  calibrated  carefully  from  a  scratch  on  the 
tube  near  the  bottom  of  that  portion  which  would  be  filled 
with  gas.  Curves  were  plotted  and  the  irregularities  ex- 
pressed in  "calibration  units."2  Next  the  capillary  de- 
pression of  the  tube  was  determined  as  some  point,  and  then 
the  tube  was  carefully  cleaned  again,  dried  and  filled  in  the 
customary  manner  with  nitrogen.  The  next  step  in  the  pro- 
cedure was  the  determination  of  the  volume  of  gas,  expressed 
in  calibration  units,  contained  in  the  closed  manometer. 
Three  methods  were  used — two  differing  only  slightly  from 
each  other.  One  method  was  to  place  the  manometer  in  a 
steel  block  and  calculate  the  volume  from  the  known  volume  of 
a '  'standard"  manometer.  The  '  'steel  block' '  is  nothing  more 
than  a  strong  reservoir  with  receptacles  for  three  manometers 
and  plungers  with  which  to  secure  a  wide  range  of  pressures. 
From  the  known  volume  of  the  "standard"  one  could  cal- 
culate the  pressure  its  gas  volume  was  under.  Then  with 
the  proper  corrections  applied  to  the  manometer  under 
comparison  one  could  calculate  the  volume  of  gas  enclosed 
at  o0-y6o  mm.  This  method  soon  fell  under  suspicion  for 
reasons  not  fully  understood  at  the  time — these  will  be  pre- 
sented later.  The  other  method  consisted  of  the  open 
"side  tube"  method.  The  "side  tube"  consisted  of  a  portion 
of  the  same  capillary  from  which  the  manometer  was  made. 
The  purpose  was  to  eliminate  capillary  depression  effects. 
If  the  side  tube  was  substituted  in  the  steel  block  in  place 

1  The  term  "capillary"  referred  to  manometer  tubing  in  this  paper 
means  a  tube  varying  in  diameter  from  0.4  mm.  to  0.8  mm. 

2  The  "  calibration  unit "  is  the  average  volume  of  each  millimeter  of 
the  calibrated  tube,  and  is  calculated  from  the  weight  of  a  thread  of  mer- 
cury which  fills  the  whole  length  of  the  capillary. 


of  the  ''standard  manometer"  the  pressure  on  the  manometer 
under  examination  was  easily  obtained.  It  would  be  the 
sum  of  the  height  of  the  mercury  column  in  the  side  tube, 
above  that  in  the  manometer,  plus  the  barometric  pressure. 
Knowing  the  pressure  and  observing  the  volume  of  the  gas 
in  the  manometer  and  keeping  the  latter  at  a  strictly  constant 
temperature,  it  is  only  necessary  to  apply  the  gas  law  equations 
to  find  the  volume  under  standard  conditions.  A  modifica- 
tion of  the  ''side  tube"  method  was  to  use  a  tape  wound 
rubber  tube  as  the  connecting  reservoir  between  "side  tube" 
and  manometer.  This  economized  time,  and  is  equally 
accurate.  After  a  number  of  observations  had  been  made 
at  different  pressures  on  the  manometer,  under  examination, 
the  average  value  of  these  was  assumed  to  closely  approxi- 
mate its  volume.  The  final  test  of  the  manometer  however 
came  after  it  had  registered  constant  pressure  when  set  up 
in  solutions  of  known  pressure  capacity,  i.e.,  in  terms  of  the 
manometers  chosen  before  as  probably  the  most  accurate 
ones.  That  there  was  error  in  the  absolute  gas  volumes 
of  these  manometers,  and  therefore  error  in  the  pressures 
they  registered,  was  recognized.  But  these  errors  were  small 
and  of  such  a  nature  that  they  exercised  negligible  effect  upon 
the  ratio  expressing  relation  of  osmotic  pressure  to  tem- 
perature or  in  other  words  the  temperature  coefficient. 

A   NEW  TYPE   OF   MANOMETER,   AND   DISCUSSION  OF  THE   SAME. 

However,  when  the  determination  of  absolute  osmotic 
pressure  is  to  be  considered — or  in  other  words  the  relation 
of  concentration  to  pressure,  these  errors  assume  an  alto- 
gether serious  aspect.  And  while  the  errors  are  small,  and 
the  labor  of  eliminating  them  is  tedious  and  time  consuming, 
still  the  end  hoped  for  is  worthy,  or  even  necessary.  The 
work  herein  described  has  been  an  effort  to  eliminate  as  far 
as  possible  this  last  error  source — the  manometer  error  factor. 

With  a  manometer  of  large  volume  the  majority  of  error 
sources  are  no  larger  than  in  those  of  small  volume.  Hence 
in  those  of  large  volume  the  percentage  error  would  be 
greatly  reduced.  With  this  idea  in  mind  a  new  type  of 


Fig.  I. 


Fig.  II. 


manometer  was  devised.  Figs.  I  and  II  show  the  old  and 
new  type  respectively.  The  type  shown  in  Fig.  II  has  a  tube 
of  large  bore  sealed  between  the  two  portions  of  small  bore. 
The  purpose  of  this  is  to  furnish  a  large  volume  without 
unduly  lengthening  the  instrument.  These  large  tubes  are 
so  selected  as  to  length  and  bore  that  the  instruments  may 
be  used  only  above  certain  concentrations,  that  is,  a  solution 
must  exert  sufficient  pressure  to  sustain  the  mercury  at  a 
point  several  millimeters  above  the  joint  of  the  enlarged 
portion  of  the  tube  and  the  capillary  above.  The  pressures 
necessary  vary  from  3  or  4  to  20  atmospheres  for  different 
manometers. 

The  capillary  tubes  were  selected  with  the  usual  care  and 
put  into  the  hands  of  an  expert  glass  blower.  They  were 
returned  as  straight  tubes,  extending  in  length  a  few  centi- 
meters below  bulb  3  and  not  sealed  at  the  top  as  shown  at  4. 
These  tubes  were  carefully  annealed,  allowed  to  rest  un- 
disturbed for  some  months  before  calibration.  These  tubes 
were  then  carefully  calibrated  by  the  usual  method  in  use 
in  this  laboratory.1  From  Fig.  II  it  may  be  observed  that 
small  marks  cross  the  capillary  a  few  millimeters  above 
bulb  3  and  the  enlarged  portion  of  the  tube.  These  marks 
are  known  as  the  "lower  scratch"  and  "upper  scratch" 
respectively,  and  the  small  capillaries  above  them  are  known 
as  the  "short  and  "long"  capillaries  respectively.  The 
"long  capillary"  was  first  calibrated  and  the  "calibration 
unit"  determined.  Next  the  "short  capillary"  was  cali- 
brated and  its  curve  plotted  in  terms  of  the  long  capillary's 
calibration  unit.  Now  a  thread  of  mercury  was  run  into 
the  enlarged  portion  of  the  tube  until  it  filled  the  tube  from 
the  upper  scratch  to  some  point  certainly  within  the  cali- 
brated portion  of  the  lower  capillary.  The  length  was  ob- 
served, the  temperature  likewise  and  the  mercury  was  run 
out  and  weighed.  From  the  data  in  hand  the  total  volume 
between  scratches  was  obtained,  and  this  in  turn  was  ex- 
pressed in  the  calibration  unit  of  the  long  capillary.  This 
latter  operation  was  all  gone  over  in  duplicate  by  the  writer, 

1  This  work  was  done  by  Dr.  Holland. 


10 

and  the  values  obtained  agreed  very  precisely  with  the  former 
determinations. 

DETERMINATION   OF    CAPILLARY    DEPRESSION. 

(a)   Purification  of  Mercury. 

The  next  examination  to  be  carried  out  was  a  detailed 
one  for  capillary  depression.  For  this  purpose  and  also  for 
filling  the  manometers,  a  supply  of  very  pure  mercury  was 
necessary.  The  smallest  amount  of  impurity  either  dissolved 
or  mechanically  held  is  well  known  to  seriously  hamper  the 
free  flow  of  mercury  through  tubes  of  small  bore.  Hence 
the  necessity  of  very  pure  material.  This  had  been  pre- 
viously, prepared,  and  the  preparation  had  best  be  described 
here.  To  obtain  pure  mercury  is  no  such  simple  matter  as 
might  appear  to  the  inexperienced.  Too  great  care  cannot 
be  exercised  in  its  preparation.  A  high  grade  mercury  was 
obtained  from  dealers  and  was  subsequently  treated  by  the 
writer  in  the  following  manner,  (i)  about  four  to  six  pounds 
were  placed  in  a  long-necked,  hard  glass  receiving  bulb.  This 
had  fitted  into  the  neck  a  two  hole  cork  stopper  carrying  two 
glass  tubes,  one  of  which  extended  below  the  surface  of  the 
mercury  and  opened  into  the  air  without,  the  other  extended 
just  below  the  stopper  and  at  the  other  end  was  attached 
to  a  pump.  The  bulb  was  placed  on  a  sand  bath  and  heated 
to  the  boiling  point  of  mercury  for  four  to  six  hours,  air  being 
drawn  through  the  entire  time.  The  effect  was  surprising 
in  that  a  large  amount  of  impurity  was  oxidized  and  appeared 
on  the  surface  as  scum.  (2)  After  cooling,  and  filtering 
through  a  paper  perforated  with  pin  holes,  the  now  bright 
metal  was  introduced  into  and  distilled  through  a  vacuum 
still.  This  still  was  of  a  simple  order,  being  made  from 
a  piece  of  Carius  combustion  tubing.  The  supply  reservoir 
of  the  still  was  an  ordinary  U-tube  and  from  this  the  supply 
arm  led  up  to  the  still  proper.  This  tube  was  of  such  a 
length  that  by  merely  raising  or  lowering  the  U-tube  one 
could  adjust  the  length  of  the  mercury  column  to  meet  baro- 
metric changes.  The  delivery  tube  was  somewhat  more  than 
barometric  length  and  hence  always  maintained  a  column 


II 

of  mercury  sufficient  to  offset  atmospheric  pressure.  The 
upper  portion  of  the  delivery  tube  was  somewhat  enlarged 
so  that  the  mercury  would  have  greater  condensing  surface, 
and  so  that,  in  falling,  air  was  continually  being  trapped  and 
carried  out.  Once  the  still  was  in  operation  it  gradually  kept 
refining  its  own  vacuum.  (3)  The  mercury  was  next  washed 
by  the  method  of  Lothar  Meyer.  Instead  of  ferric  chloride 
solution,  however,  a  two  per  cent,  nitric  acid  and  two  per 
cent,  mercurous  nitrate  solution  was  used.  A  very  effective 
means  of  breaking  the  mercury  into  fine  globules  was  em- 
ployed by  the  use  of  a  silk  bolting  cloth  strainer.  A  half 
liter  separatory  funnel  was  flared  somewhat  at  the  delivery 
stem  and  over  this  was  bound  a  double  thickness  of  the  cloth. 
On  opening  the  stopcock  and  allowing  the  mercury  to  enter 
it  breaks  into  perhaps  many  thousands  of  fine  globules. 
In  fact  the  separation  is  so  effective  that  the  whole  length 
of  the  two  meter  verticle  tube,  through  which  it  falls,  is  darkly 
and  heavily  clouded.  This  exposes  an  enormously  large 
surface  to  the  action  of  the  acid  and  salt.  Futhermore 
the  pouring  was  repeated  1000  times.  By  continually  re- 
newing the  solution  it  would  seem  quite  safe  to  think  that 
all  those  metals  which  are  volatile  with  mercury  vapor,  and 
hence  had  not  been  left  behind  in  process  (2)  would  be  re- 
moved. (4)  After  washing  with  water,  drying  and  filtering 
again,  the  mercury  was  finally  redistilled  through  a  second 
vacuum  still.  This  still  was  of  the  same  type  as  the  one 
described  above,  but  very  much  smaller.  Before  use  in  a 
manometer  this  mercury  was  filtered  again,  either  through 
hard  filter  paper  perforated  with  pin  holes,  or  through  a  clean 
funnel  drawn  to  a  capillary  at  the  end.  The  latter  has  the 
advantage  that  it  offers  no  lint  or  dust  to  stick  to  the  surface 
of  the  metal.  An  entirely  satisfactory  grade  of  mercury 
was  thus  obtained. 

(b)  Discussion  of  Capillary  Depression. 

The  fact  that  it  has  been  found  most  practicable  to  de- 
termine manometer  volumes  at  low  pressures  caused  the 
capillary  depression  factor  to  assume  altogether  important 


12 

proportions.  As  mentioned  above  it  was  sought  to  escape 
this  factor,  in  so  far  as  it  affected  volume  determinations, 
by  the  use  of  the  ''side  tube"  cut  from  the  same  piece  of  tubing 
as  was  the  manometer.  That  it  did  not  eliminate  the  error, 
but  probably  introduced  one  will  be  shown  later. 

The  procedure  of  determining  the  capillary  depressions 
was  very  simple.  A  tube  40  mm.  in  diameter  and  25  cm. 
long  was  used  as  the  reservoir.  This  was  sealed  to  a  short 
piece  of  ordinary  thick-walled  barometer  tubing,  and  manom- 
eter and  reservoir  were  connected  by  a  suitable  length  of 
tape-wound  rubber  tubing  filled  with  mercury.  Before 
attaching  the  rubber  tube  to  the  manometer  the  latter  was 
supplied  with  a  sufficient  quantity  of  the  pure  mercury  to 
render  it  certain  that  the  meniscus  would  be  clean  and  the 
flow  of  the  mercury  free.  The  point  at  which  the  meniscus 
stood  could  be  varied  at  will  by  raising  or  lowering  the 
reservoir. 

This  work  was  carried  out  in  the  "Manometer  house," 
which  is  kept  at  a  constant  temperature — and  will  be  described 
later.  Hence  the  probability  of  error  from  fluctuating  tem- 
perature-was avoided. 

Since  the  effect  of  capillary  depression  was  most  serious 
as  to  its  bearing  upon  volume  determinations,  and  since  the 
meniscus  in  the  manometer  would  stand  somewhere  in  the 
short  capillary  while  its  volume  was  being  determined,  it 
is  at  once  apparent  that  a  detailed  examination  of  that  portion 
of  the  manometer  should  be  carried  out.  The  capillary 
depressions  were  ascertained  at  shorter  intervals  and  with 
more  precaution  as  to  tapping  in  the  short  capillary  than  in 
the  long  capillary.  Of  course  as  the  pressure  increases  in 
a  manometer  the  capillary  depression  error  decreases,  and 
indeed  above  a  few  atmospheres  become  quite  insignificant. 

The  tendency  on  the  part  of  the  mercury  to  lag  in  the 
small  capillary  was  overcome  by  tappers.  These  were  the 
ordinary  coils  and  hammer  of  small  call  bells,  mounted  upon 
weighted  standards.  These  could  be  easily  placed  in  a 
suitable  position  and  controlled  by  a  button  on  the  outside 
of  the  manometer  house.  If  it  were  left  to  these  tappers  alone 


13 

to  establish  equilibrium  between  the  columns  it  frequently 
required  more  than  an  hour.  However,  the  operator  found 
that  a  preliminary  tap  or  two  on  the  connecting  rubber 
tube  with  a  pencil  would  so  hasten  equilibrium  that  only 
ten  to  fifteen  minutes  subsequent  tapping  by  the  hammers 
was  required. 

It  has  been  suggested  that  it  is  unnecessary  to  go  through 
the  labor  of  experimentally  determining  the  capillary  depres- 
sion of  small  bore  tubes.  That  instead  of  such  experimental 
determination  the  depressions  could  be  calculated  directly 
from  the  surface  tension  value  of  mercury  and  the  diameter 
of  the  tube,  expressed  by  the  following  equation. 

2T  2T 

dgh  =  —  or  h  =  -j— ,  where    h  would  represent    the   de- 

r  dgr 

pression,  T  the  surface  tension  of  mercury  in  dynes  per 
centimeter,  d  the  density  of  the  mercury  at  the  temperature 
observed,  g  the  gravitational  constant,  and  r  the  radius  of 
the  tube.  Such  a  calculation  would  be  entirely  satisfactory 
if  one  could  be  certain  of  the  values  of  T  and  r .  The  value 
of  T,  however,  is  so  strictly  a  function  of  the  purity  and  clean- 
liness of  the  mercury  that  it  is  not  without  risk  that  the 
commonly  accepted  values  are  adopted.  Futhermore,  no 
scruple  must  be  spared  in  cleansing  the  capillary  tubes. 
These  two  factors — the  mercury  and  the  tube — place  the 
investigator  in  a  dubious  frame  of  mind  as  to  how  well  some 
one  else's  value  for  T  will  fit  his  own  needs.  Equally  as 
serious,  or  probably  more  serious,  is  the  unreliability  of  the 
value  for  r  at  any  particular  point.  To  determine  the  di- 
ameter of  a  capillary  tube  exactly,  at  points  very  slightly 
removed  from  each  other,  through  any  considerable  length 
of  a  tube  would  involve  an  enormous  amount  of  time  and 
labor.  Slight  irregularities  only  a  millimeter  or  two  apart 
cause  very  marked  differences  in  capillary  depression,  hence 
it  is  not  without  grave  risk  that  the  mean  diameter  through 
even  a  short  distance  may  be  adopted.  Nor  is  it  in  any 
measure  safe  to  determine  the  capillary  depression  at  one  or 
two  points  and  adopt  the  mean  as  the  true  depression  for  all 
points. 


The  accompanying  Table  I  will  furnish  some  idea  of  how 
variable  depressions  may  be  at  points  not  remote  from  each 
other,  even  though  the  tube  has  been  chosen  with  utmost 
care.  The  manometer  was  of  the  old  type,  as  shown  in  Fig. 
I,  and  is,  for  certain  laboratory  convenience,  designated  M.  5. 

TABLE  I. 


Distance  abort 

Capillary 

Distance  above 

Capillarj' 

scratch. 

depression. 

scratch. 

depression. 

8.65 

7.92 

H7-43 

II  .42 

22.70 

10.85 

224.  12 

II.I8 

47-35 

9.87 

280.30 

11.74 

7I-38 

10.04 

36I.IO 

II.80 

114.28 

10.42 

414.  10 

12  .  14 

Now  since  pressures  ranging  from  800  to  1000  milli- 
meters constitute  the  limits  between  which  one  must  deter- 
mine the  volumes  of  these  manometers  if  open  side  is  used, 
any  considerable  discrepancy  in  capillary  depression  would 
have  a  grave  effect  on  the  accuracy  of  the  final  result.  In 
fact,  at  such  low  pressures,  a  difference  of  one  millimeter 
amounts  to  a  difference  of  about  one  calibration  unit  when 
the  final  volume  is  calculated.  If  in  the  case  of  manometer 
M.  5  the  depression  had  not  been  taken  nearer  the  scratch 
than  22.7  millimeters,  and  the  observations  carried  out 
from  that  point  as  tabulated,  by  former  usage  the  mean  value 
1 1.05  of  these  last  nine  observations  would  have  been  adopted. 
If  now  the  manometer  had  been  filled  with  nitrogen,  and  sealed 
and  its  volume  under  process  of  determination,  unless  suffi- 
cient pressure  were  brought  to  bear  to  bring  the  meniscus 
above  22.7  mm.  from  the  scratch  an  uncertainty  at  least 
would  be  introduced  as  to  the  final  gas  volume  in  the  manom- 
eter. If  the  meniscus  could  not  be  brought  above  9  milli- 
meters above  the  scratch,  and  if  the  mean  depression  value 
of  11.05  were  assumed  to  be  the  depression  at  that  point, 
an  error  of  11.05  —  7.92  =  3.13  calibration  units  would 
have  resulted.  The  volume  of  this  manometer,  subsequently 
determined,  was  503.00  calibration  units.  An  error  of  3.13 
calibration  units  would  have  caused  an  error  of  about  0.62 
per  cent,  on  the  volume.  While  this  error  would  have  de- 


15 

creased  as  pressure  increased  still  it  would  have  furnished 
sufficient  discrepancy  from  other  instruments  to  stimulate 
distrust  in  its  accuracy.  Hence,  as  has  been  stated  above, 
very  detailed  determinations  of  capillary  depressons  were 
made  in  the  short  capillaries  of  the  manometers,  and  care- 
ful though  less  detailed  determinations  in  the  long  capil- 
lary. Curves  were  plotted  for  depressions  in  the  same  fashion 
as  for  calibration. 

FUSING   THE    MANOMETERS. 
(a)   The  Preparation  of  the  Nitrogen. 

The  manometer  tubes  were  now  considered  ready  for 
filling  with  nitrogen.  To  the  stem  below  bulb  3,  Figs.  I 
and  II,  the  straight  tube  carrying  bulbs  i  and  2  was  sealed 
and  subsequently  bent  as  shown  in  the  figure.  To  be  quite 
satisfied  as  to  the  cleanliness  of  these  tubes — now  unfilled 
manometers — they  were  subjected  to  a  third  cleansing  with 
sulphuric  acid  chromic  acid  mixture,  washed  out  with 
distilled  water,  and  finally  washed  several  times  with  "con- 
ductivity water.".  They  were  then  placed  in  a  drying  train, 
and  dry  air  pumped  through  for  not  less  than  18  hours. 

Mention  has  been  made,  in  a  paper1  already  published, 
of  the  fact  that  air  was  at  one  time  used  to  fill  the  manom- 
eters and  that  nitrogen  was  subsequently  adopted.  When 
air,  however,  carefully  washed  and  dried,  was  used,  there 
seemed  ultimately  to  be  a  decrease  in  the  volume  of  the  gas 
in  the  manometer.  This  could  be  due  to  oxidation  of  im- 
purity in  the  mercury — as  oxidizable  impurity  might  have 
been  present.  The  adoption  of  nitrogen  has  eliminated  that 
trouble.  The  preparation  of  the  nitrogen  was  marked  by 
the  same  care  which  was  exercised  in  all  the  different  steps 
of  the  work.  The  nitrogen  was  prepared  from  air  in  the 
following  way:  The  air  was  drawn  through  a  train  of 
bottles  containing  alkali  pyrogallate,  continuing  through  a 
tube  at  red  heat  containing  reduced  copper,  then  on  through 
wash  bottles  containing  alkali  pyrogallate  and  concentrated 
sulphuric  acid  respectively,  thence  through  another  tube 

1  Loc.  cit. 


i6 

at  red  heat  containing  first  copper  oxide  wire  then  reduced 
copper  gauze,  thence  through  tubes  of  fused  calcium  chloride 
and  stick  caustic  alkali,  and  finally  through  a  tube  of  resub- 
limed  phosphorous  pentoxide  distributed  over  asbestos  fiber. 
The  whole  train  was  filled,  heated,  and  air  drawn  in  for  some 
time.  Then  it  was  closed  and  allowed  to  stand  for  some 
hours  in  order  to  give  opportunity  for  a  diffusion  from  all 
packed  places  of  any  oxygen  or  other  gas  that  could  be  sep- 
arated out.  After  reheating,  and  drawing  in  a  current  of  air 
for  some  time  and  allowing  the  nitrogen  to  waste,  the  reser- 
voir was  finally  attached  and  a  supply  of  nitrogen  collected. 

(6)  Filling  and  Closing  the  Manometer. 
The  method  of  filling  the  manometers  was  the  same  as  that 
used  in  former  work,  viz.:  the  mercury  was  first  drawn  into 
bulbs  i,  2  and  3.  Then  the  manometer  is  sealed  at  the  top 
to  a  stem  from  the  nitrogen  reservoir.  The  nitrogen  is  used 
to  wash  the  manometer  free  from  air  several  times  before  it 
is  finally  closed.  After  allowing  the  desired  quantity  of 
gas  to  enter,  the  usual  mercury  thread  is  run  in  at  the  top, 
and  the  manometer  sealed  off  as  described  in  a  paper  already 
referred  to.  Only  one  modification  of  the  process  was  adopted. 
Formerly,  when  the  top  of  the  manometer  was  being  sealed 
off,  the  short  mercury  thread  immediately  underneath  the 
play  of  the  blow  pipe  flame  often  became  so  agitated — due 
to  sudden  vaporization  and  condensation  of  its  top  portions — 
that  globules  frequently  became  detached  and  trapped  gas 
between  themselves  and  the  main  thread.  This  caused  much 
trouble,  and  frequently  required  reopening  the  manometer 
for  their  dislodgment.  This  was  remedied  by  simply  soften- 
ing the  walls  of  the  tube,  somewhat  above  that  portion  at 
which  the  upper  meniscus  would  stand,  and  allowing  the 
capillary  to  become  constricted  to  very  fine  bore.  This 
measure  hindered  the  rapid  vibration  back  and  forth  of  the 
column  and  not  a  single  accident  was  experienced  in  the 
entire  lot  of  manometers. 

DETERMINATION  OF  GAS  VOLUMES. 

Since  the  object  of  this  entire  investigation  was  the  develop- 


Fig.  III. 


18 

ment  of  a  reliable  method  for  manometer  construction,  and 
since  up  to  this  stage  of  the  work  no  pains  or  time  had  been 
spared  for  the  sake  of  accuracy,  it  was  necessary  here  to  hit 
upon  the  most  reliable  method  of  work  also.  Much  care  was 
taken  to  arrange  a  bath  which  would  conveniently  accommodate 
all  the  necessary  apparatus,  and  at  the  same  time  be  easily 
regulated  at  a  constant  temperature.  This  bath  is  known 
as  the  "manometer  house."  All  the  essential  details  are 
shown  in  Fig.  III.  In  the  figure  (i)  is  a  shelf  on  which  is 
arranged  all  the  instruments,  such  as  the  "steel  block"  the 
"brass  block,"  the  meter  scale,  the  tappers,  etc.  This  shelf 
rests  on  heavy  brackets  which  are  bolted  to  the  heavy  masonry 
of  the  wall  behind.  The  thermoregulator  is  hung  from  a 
bracket  on  the  wall  itself  and  is  thus  freed  from  the  effects 
of  the  vibrations  caused  by  the  tappers  when  these  are  at 
work.  Partition  (2)  merely  serves  to  hold  back  the  air  and 
necessitates  its  being  forced  by  the  fan  motor  through  the 
hole  in  the  partition.  The  arrows  show  its  direction  of  cir- 
culation. At  each  end  of  shelf  (i)  a  5  cm.  space  allows  the 
air  to  pass  through  the  upper  compartment.  This  bath 
is  kept  a  few  degrees  above  the  temperature  of  the  outside 
room — this  latter  being  regulated  roughly  by  a  steam  radi- 
ator or  a  gas  stove.  Control  of  temperature  is  maintained 
by  the  lamps  shown  in  the  figure — the  lamps  being  controlled 
by  the  thermoregulator  above.  In  fact  the  system  of  the 
electric  control  of  heat  is  identical  with  that  in  use  in  all  the 
constant  temperature  baths  employed  in  this  work.  With 
this  arrangement,  fluctuation  of  temperature  is  kept  well 
within  o.i  of  a  degree.  More  minute  details  of  the  manom- 
eter house  are  shown  from  Fig.  IV. 

The  steel  block,  which  has  been  previously  described,  was 
orginally  designed  for  the  determination  of  manometer  gas 
volumes  against  a  "standard"  manometer.  The  volume 
of  the  proposed  standard  was  determined  at  low  pressures — 
but  without  the  precautions  observed  in  the  recent  work. 
The  idea  was  to  find  the  means  of  comparing  other  manom- 
eters under  high  pressure  with  this  one.  Assuming  that 
the  original  volume  of  the  standard  was  correct,  the  pressure 


19 


it  would  be  under  at  any  position  of  the  meniscus  could  be 
easily  calculated  from  the  gas  laws.  Further,  if  another 
manometer  was  in  series  with  it,  if  proper  corrections  were 
applied  for  the  different  heights  of  mercury,  the  pressures 
must  have  been  identical.  From  this  pressure  the  volume 
of  the  second  manometer  was  in  turn  calculated.  No  satis- 
factory results  were  ever  obtained  by  the  "standard"  manom- 
eter method.  At  low  pressures  a  different  volume  was 
always  calculated  than  at  high  pressures.  In  Fig.  V  is  shown 


95 

93 
92 
9/ 
290 
69 
fia 
87 
66 

as 

A4 
62 

a/ 
zao 
m 

F/G.V. 

i 

/ 

1  l/Q/l/THf*  IT)  C*  olihraT/o-n  /Vn/'/fe-  VdT/af/O'ns. 

/ 

Tvaicof    Curve  ^or    Volume   o/  fl    /ManoTVeter- 

/ 

£?, 

t&rm  nt 

*d     Aaa 

in 

*t  ft,  e     "  Sf< 

•^-ndard" 

/ 

/ 

^ 

> 

^ 

/ 

X 

^ 

X 

^ 

^ 

^^ 

5 

^ 

-^ 

£ 

-^ 

\ 

/ 

x 

x 

?r 

<"*) 

SU 

re 

s 

i'rt 

/\rrno 

^fi 

he 

re 

^ 

a  typical  curve  for  the  "volume"  of  nitrogen  in  a  manometer 
determined  by  this  method.  Obviously,  a  volume  curve 
of  a  closed  manometer  is  a  straight  line.  Hence  some  con- 
stant error  was  affecting  the  results.  It  is  more  probable 
that  two  factors  were  at  work.  That  one  which  was  most 
effective  under  low  pressure  was  probably  due  to  capillary 
depression.  That  under  high  pressure  appears  to  have 
been  due  to  error  in  meniscus  correction.  The  former  error 
would  decrease  in  importance  as  the  pressure  was  increased, 
while  the  latter  error  would  increase  in  marked  proportion 
with  increased  pressure. 

The  proper  meniscus  correction  has  been  a  much  mooted 
question,    and   confession   must   be   made   that  no   method 


20 

without  theoretical  or  practical  objection  is  at  hand.  It 
is  stated  that  perfectly  pure  mercury  leaves  perfectly  pure 
glass  at  an  angle  of  148  degrees.  The  determination  of  such 
an  angle  would  not  be  difficult  from  a  plane  surface,  or  from 
a  cylindrical  surface  provided  the  arc  of  curviture  of  the  latter 
was  sufficiently  great  to  furnish  a  tube  of  such  diameter  that 
capillary  effects  were  overcome;  but  in  a  tube  of  very  small 
bore  such  determination  would  be  a  difficult  matter.  To 
assume  that  the  meniscus  in  tubes  of  small  bore  is  hemi- 
spherical, approaches  the  true  state  of  affairs  but  falls  somewhat 
short  of  the  mark.  However,  we  have  found  the  correction 
based  on  such  an  assumption  to  be  perhaps  the  best  avail- 
able, and  such  correction  has  been  applied  in  this  work. 
The  method  follows:  "Assuming  that  the  surface  of  a  meniscus 
is  similar  in  shape  to  half  a  sphere  and  that  the  two  surfaces 
of  a  double  meniscus  are  equivalent  to  an  entire  spherical 
surface  the  following  conclusions  may  be  drawn :  In  a  manom- 
eter there  are  a  number  of  cylinders,  each  capable  of  con- 
taining a  definite  number  of  cubical  units,  the  number  of 
which  would  depend  upon  the  position  of  the  menisci  at  the 
time  a  volume  was  included. 

"Since  the  volume  of  a  cylinder  =  it  ~R2h,  where  R  = 
radius  of  the  circular  section,  and  h  =  the  height;  and  since 
the  volume  of  a  sphere  =  4/3  n  R3,  and  considering  the 
sphere  incribed  in  the  cylinder,  then  the  volume  of  the  double 
meniscus  is  equivalent  to  the  difference  in  volume  of  the 
cylinder  and  the  inscribed  sphere.  n  R2&  —  4/3  n  R3  =  x\ 
h  =  2R.  Then  n  R2/t  =  2  n  R3;  2  n  R3  —  4/3  *  R3  = 
2/3  n  R3  =  volume  of  double  meniscus. 

"Now  since  this  expression  above  is  in  cubical  units  and 
the  volume  of  the  manometers  is  expressed  in  linear  units, 
the  former  must  be  converted  also  into  linear  units. 

"Now  2/s  n  R3  =  volume  in  cubical  units 

TT  R2  =  area  in  cross  section 

2/3  TT  R3  divided  by  n  R2  =  2/3  R  =volume  in 
linear  units.  2/3  R  =  l/3T>.  Hence,  by  determining  the 
mean  diameter,  the  meniscus  correction  is  at  once  obtained." 

By  a  recent  recalculation  of  a  series  of  volume  determina- 
tions on  a  manometer  which  was  compared  with  the  "stand- 


21 

ard,"  upon  application  of  the  revised  meniscus  correction, 
the  curve  shown  in  Fig.  V  was  straightened  out  somewhat 
at  higher  pressures.  This  seemed  to  fix  the  trouble  upon 
that  factor.  The  importance  of  the  meniscus  correction  may 
be  seen  from  the  following  extract  from  a  paper1  on  this 

work:" in    determining    the    temperature    coefficient, 

the  errors  in  the  meniscus  correction,  if  they  are  uniform, 
may  be  very  large  without  seriously  affecting  the  result; 
but  when  it  is  attempted  to  ascertain  the  relation  of  osmotic 
pressure  to  concentration,  the  case  is  very  different,  for 
then  the  pressures  of  all  the  various  concentrations 
of  solution  are  to  be  compared  at  fixed  temperatures, 
and  the  meniscus  corrections  have  consequently  widely 
differing  values.  This  is  illustrated  from  the  data  taken 
from  the  record  of  a  single  manometer  (No.  9) .  The  meniscus 
correction  (double)  in  this  instrument  is  0.17  calibrations 
unit,  and  the  volume  of  nitrogen  under  standard  conditions 
of  temperature  and  pressure  is  454.14  calibration  units. 
Column  I  in  the  table  gives  the  weight  normal  concentration 
of  the  solutions;  II,  the  pressures  in  atmospheres;  III,  the 
volume  of  the  compressed  nitrogen  reduced  to  standard 
temperature;  IV,  the  corrections  in  fractions  of  an  atmos- 
phere for  the  double  meniscus;  V,  the  relative  osmotic  pres- 
sures, the  pressures  of  the  o.i  normal  solution  serving  as  the 
unit.  Column  VI  contains  the  relative  corrections  for 
meniscus,  the  correction  for  the  o.i  normal  solution  serving 
as  the  unit.  The  temperature  in  all  cases  is  25  degrees. 


I 

• 

Os. 

II. 
pres. 

in. 

Vol.  nitrogen 

IV. 
Men.  cor. 

V. 
Rel.  os. 

VI. 
Rel.  men. 

:onc.    atmos. 

cal  units. 

atmos. 

pres. 

cor. 

0, 

I 

2 

.635 

141 

•15 

o 

.00317 

I 

.OOOO 

I 

.OOOO 

O, 

2 

5 

•139 

80 

.69 

o 

.01083 

j 

9503 

3 

.4164 

0. 

o 

7 

•738 

55 

•59 

0.02566 

2 

.9366 

7 

.4637 

0. 

4 

10 

•295 

42 

.41 

0. 

02126 

O 

9070 

13 

0158 

o, 

5 

12 

947 

34 

.01 

0 

06972 

4 

9135 

21 

9937 

0 

6 

15 

.620 

28 

•37 

o 

09360 

5 

9275 

29 

.5268 

0 

7 

18 

•436 

24 

.  ii 

0 

12999 

6 

9928 

41 

.0063 

0 

8 

21 

.258 

20 

•97 

0 

17233 

8, 

1055 

ZA 
O'T 

.3628 

o 

9 

24 

.126 

18 

•53 

o. 

22133 

9 

1558 

69 

,8202 

I 

0 

27 

.076 

16 

•54 

o, 

27834 

10. 

2755 

87 

,  8044 

1  Am.  Chem.  Jour.,  45,  237. 


22 

Particular  attention  is  called  to  columns  V  and  VI,  where 
it  will  be  seen  that,  while  osmotic  pressure  increased  a  little 
over  ten  fold,  the  value  of  the  meniscus  correction  increased 
nearly  88  fold." 

As  was  stated  earlier  in  this  discussion  of  the  standard, 
no  means  was  ound  of  avoiding  discrepancies  in  volumes 
of  compared  manometers  under  low  pressures.  It  is  reason- 
ably certain  however  that  these  are  due  to  variations  of 
capillary  depressions.  And  since  no  way  of  ascertaining 
these  was  at  hand  without  opening  the  manometer  and 
experimentally  determining  the  depressions  the  "standard" 
was  discarded. 

The  next  method  used  was  that  of  "side  tube."  This 
side  tube  consisted  of  a  piece  of  tubing  cut  from  the  same 
piece  of  tubing  as  the  manometer  itself,  the  object  being,  as 
has  been  stated,  to  avoid  capillary  depression  corrections. 
In  some  instances  concordant  volumes  were  obtained  from 
several  observations.  The  range  of  pressures  was  neces- 
sarily limited,  as  has  been  explained  before.  But  even 
through  this  narrow  range  of  pressures,  too  wide  variations 
were  frequently  found  to  be  ascribed  to  errors  of  observation. 
It  was  found  that  the  errors  of  capillary  depression  in  the 
manometer  tube  and  the  side  tube  of  presumably  the  same 
bore  were  in  a  great  majority  of  cases,  if  not  in  fact  always 
additive.  In  such  a  case  as  that  recited  in  Table  I,  errors 
arising  in  this  way  would  render  the  manometer  quite  worth- 
less, and  the  time  spent  in  its  preparation,  as  well  as  that  in 
its  use,  would  be  lost. 

In  order  to  avoid  such  liability  to  error  in  determining  the 
volume  of  nitrogen  under  standard  conditions,  a  side  tube 
40  mm.  in  diameter  was  adopted — the  same  tube,  in  fact, 
that  was  used  in  capillary  depression  determinations.  One 
of  these  tubes  was  sealed  on  to  a  barometer  tube  of  ordinary 
bore  and  about  60  cm.  length.  The  tube  carried  a  stopcock 
so  that  the  column  of  mercury  might  be  maintained  while 
changing  manometers  in  the  block.  This  tube  is  represented 
in  Fig.  III.  Results  obtained  by  use  of  this  tube  were  highly 
satisfactory.  Two  objections  arose  to  its  use  however. 


23 

It  was  impossible  to  vary  the  pressure  even  a  few  millimeters ; 
and  furthermore  the  constant  insertion  and  removal  of 
manometers  to  and  from  the  '  steel  blocks"  augmented  the 
chance  of  accident  and  breakage,  further  the  process  was 
very  slow.  The  method  which  proved  most  satisfactory  of 
all,  and  that  which  was  adopted,  was  the  use  of  the  wide 
side  tube,  by  rubber  tube  connection  with  the  manometer. 
In  fact  the  same  fashion  after  which  capillary  depression  was 
determined — the  difference  being  that  now  the  manometer 
was  closed  where  before  it  was  open .  Comparisons  were  made  on 
manometers  by  the  side  tube  method,  using  the  capillary  tube 
of  presumably  the  same  bore  as  the  manometer,  both  in  the 
steel  block  and  by  rubber  tube  connection,  against  the  values 
obtained  when  the  wide  side  tube  was  used  both  in  the  steel 
b:ock  and  by  rubber  tube  method.  In  the  case  of  the  narrow 
side  tube  no  satisfactory  agreements  in  volume  from  several 
observations  at  different  pressures  could  usually  be  obtained. 
With  the  wide  side  tubes  the  agreement  was  quite  satisfactory 
both  in  the  block  and  rubber  tube,  and  furthermore  these 
values  agree  closely  with  each  other.  Having  established 
the  reliability  and  expediency  of  the  method,  work  was  im- 
mediately carried  through  on  the  entire  lot  of  the  new  type 
manometer.  The  procedure  was  briefly  as  follows.  Manom- 
eter and  side  tube  were  connected  by  means  of  the  rubber 
tube.  (In  order  to  avoid  refilling  the  side  tube  and  rubber 
tube  at  each  change  of  manometers,  a  screw  clamp  was  used 
to  close  the  rubber  near  the  manometer  stem  before  remov- 
ing the  latter.)  In  this  operation  care  was  taken  to  admit 
no  air  into  the  stem  of  the  manometer.  Both  side  tube  and 
manometer  were  clamped  firmly  into  position,  and  small 
mirrors  bound  on  at  a  suitable  angle  for  reflection  of  light  to 
the  telescope.  A  tapper  was  placed  in  position  against  the 
manometer;  and  the  whole  system  was  then  allowed  to  come 
to  constant  temperature  of  the  bath.  After  temperature 
equilibrium  had  certainly  been  established,  and  after  ef- 
fective tapping  of  the  manometer,  the  following  observations 
were  taken:  Volume  of  gas  in  manometer  between  the  two 
menisci,  barometric  pressure  and  temperature,  height  of 


24 

mercury  column  in  the  side  tube  above  the  lower  meniscus 
in  the  manometer,  and  the  constant  temperature  of  the 
manometer  house.  After  making  the  proper  corrections  and 
applying  the  gas  laws,  the  volume  of  gas  contained  in  the 
manometer  under  standard  conditions  of  temperature  and 
pressure  was  obtained.  The  pressure  on  the  manometer 
could  be  varied  in  a  limited  degree  by  raising  or  lowering  the 
side  tube.  This  fact  was  of  prime  importance,  for  it  made 
it  quite  possible  to  bring  the  lower  meniscus  to  a  portion  of 
the  capillary  in  which  its  depression  was  accurately  known. 
On  such  days  when  much  fluctuation  of  the  barometer  took 
place,  it  was  difficult  to  control  the  exact  position  of  the 
meniscus,  for  as  atmospheric  pressure  increased  the  mercury 
column  in  the  manometer  would  rise,  and  contrary,  when  the 
pressure  of  the  atmosphere  decreased,  the  mercury  column 
in  the  manometer  would  fall.  In  such  cases  the  curves  plotted 
for  the  depressions  usually  furnished  the  necessary  data  for 
the  corrections.  However,  in  a  few  instances  these  failed. 
The  latter  fact  was  probably  due  to  some  variation  of  depres- 
sion which  had  still  escaped  the  very  detailed  examination 
for  the  same.  In  such  cases  the  side  tube  was  raised  or  lowered 
until  the  meniscus  in  the  manometer  stood  in  a  portion  of 
the  capillary  where  the  depression  was  more  constant. 

With  the  exception  of  two  of  the  manometers  of  the  new 
type,  all  gave  volumes,  under  standard  conditions,  of  more  than 
1000  calibration  units.  And  the  largest  volume  of  all  was 
somewhat  more  than  3200  units.  In  the  case  of  the  old  type 
of  manometer  it  was  possible  to  obtain  at  most  only  about 
800  units,  and  in  most  cases  the  volumes  ranged  from  three 
to  five  hundred  units.  Since  the  calibration  unit  is  a  linear 
unit,  and  not  a  cubical  unit,  it  is  obvious  that  the  size  of  the 
capillary  controlling  the  calibration  unit  would  have  no 
influence  on  the  number  of  units,  in  so  far  as  increasing  or 
decreasing  that  number  is  concerned,  if  the  bore  was  of  the 
ordinary  uniformity.  Hence  it  was  impossible  to  increase 
the  number  of  units  greatly  except  by  lengthening  the  tube. 
This  was  impracticable,  and  was  rendered  unnecessary  by 
the  adoption  of  the  new  type  of  instrument. 


25 

In  determining  the  volumes  of  manometers,  it  is  impossible 
to  get  values  agreeing  any  more  closely,  in  fractions  of  cali- 
bration units,  with  the  old  type  of  manometer  than  with 
the  new  type.  Hence,  if  the  actual  fractional  agreement  is 
as  close  in  a  manometer  of  large  volume  as  in  that  of  one  of 
small  volume,  the  percentage  error  in  the  one  of  large  volume 
is  much  smaller  than  that  in  one  of  small  volume.  In  most 
of  these  manometers,  extreme  variation  of  the  maximum  or 
minimum  volume,  determined  by  observation,  from  the  mean 
of  all  the  values  varied  from  0.03  to  o.i  per  cent.  The 
variations  fall  well  within  the  limits  of  errors  of  observation. 

COMPARISON    OF    MANOMETERS. 

One  more  step  was  taken  before  the  manometers  were 
considered  ready  for  osmotic  pressure  work.  And  this  step 
would  prove  the  worth  or  unfitness  of  the  instruments.  They 
were  compared  against  each  other.  That  is  to  say,  two  manom- 
eters were  connected  in  such  a  way  that  their  gas  volumes 
were  under  the  identical  compression.  The  comparisons  to 
the  present  have  been  made  at  low  pressures  only. 

In  order  to  compare  two  manometers  against  each  other,  at 
low  pressures,  they  were  connected  by  the  same  rubber  tube 
used  in  the  methods  for  determination  of  capillary  depressions 
and  volumes.  Instead  of  side  tube  and  manometer,  there  was 
now  placed  manometer  and  manometer.  The  same  pre- 
cautions were  used,  as  before,  to  see  that  the  rubber  tube  and 
stems  of  manometers  were  entirely  filled  with  mercury  and 
free  of  air  bubbles.  By  adjustment  of  one  of  the  manometers, 
up  or  down,  the  meniscus  of  each  could  usually  be  brought 
to  a  desired  point  on  the  short  capillary.  Tappers  were  set 
is  position  against  the  instruments  and  the  system  allowed 
to  come  to  the  constant  temperature  of  the  manometer 
house.  The  procedure  of  observations  was  identical  with 
that  in  the  determination  of  volume,  except  no  barometer 
readings  were  necessary. 

Obviously,  when  corrections  for  difference  in  height  of 
the  mercury  columns  in  the  two  manometers,  and  capillary 
depressions,  are  applied,  the  pressures  on  both  volumes  of 


26 

gas  must  be  identical.  And  if  the  values  for  the  volumes, 
obtained  by  the  method  set  forth  in  this  paper,  are  correct,  then 
the  calculated  pressures  from  these  volumes  should  be  iden- 
tical. 

Those  three  manometers  which  showed,  at  once,  smallest 
variation  in  calibration  corrections,  capillary  depression,  and 
percentage  variation  from  mean  in  volume,  were  chosen  as 
standards  of  comparison.  These  three  are  known  respec- 
tively as  numbers  31,  40  and  41. 

In  Table  II  is  shown  a  series  of  results  obtained.  The 
columns  respectively  represent  the  following:  (i)  the  number 
of  the  manometer;  (2)  the  value  of  the  gas  volume  in  the 
manometer  under  standard  conditions,  the  value  being 
obtained  by  the  method  described  in  this  paper;  (3)  the  ob- 
served volume  of  the  slightly  compressed  gas;  (4)  the  cor- 
rected pressure  in  millimeters  calculated  upon  the  assumption 
that  the  value  in  column  2  was  correct;  (5)  corrected  pressure 
expressed  in  atmospheres;  (6)  the  volume  at  standard  con- 
ditions, calculated  from  pressure  of  the  other  manometer  as 
a  standard;  (7)  percentage  expression  of  variation  from 
original  value  of  volume;  (8)  percentage  error  distributed 
between  the  two  manometers. 

It  will  be  noticed  in  the  following  table  that  manometer  32 
was  compared  against  each  of  the  three  chosen  standards. 
It  agrees  quite  exactly  with  numbers  40  and  41,  and  agrees 
very  closely  with  number  31.  Furthermore  numbers  31  and 
40  were  compared  against  each  other  and  agree  very  satis- 
factorily. Such  a  method  of  procedure  makes  it  possible  to 
place  all  the  instruments  on  the  same  basis. 

Seventeen  of  the  new  type  manometers  were  carried  through 
the  operations  described  in  this  paper.  Out  of  the  seventeen, 
two  (Nos.  25  and  30)  are  looked  upon  with  suspicion.  In 
Table  II  is  given  the  reason  for  suspecting  some  error  in 
number  30.  The  volume,  as  calculated  for  column  6,  is  far 
too  small  to  be  accounted  for  through  experimental  error, 
and  since  number  41  is  in  such  close  accord  with  the  other 
manometers,  it  is  but  the  natural  and  necessary  course  to 
regard  the  value  in  column  2  for  manometer  30  as  wrong. 


0      M 

Co  CO 
to   i—  i 

4"  Co 

M      tO 

4^  Co 

O      M 

Co  4*. 

ON  M 

Oo  4^ 
to    O 

Co  Co     M 

N^         ^ 

•^4   "ON 

to    to 

ON  to 
•<!     M 

ON  tO 

M      tO 

0    0 
Co  4i. 

CO    ON 
0  <1 
to    ON 

tO      M 

M      0 

to  co 

vO    to 

Cn    ON 
Cn  4^ 

OOCn 
COCO 

ON  00 
4*    00 

^4  Cn 

Cn  Co 

M    ON 

00-^1 

OOCn 

ONCn 
ON  Co 

to    ON 

vO  vo 

M    0 
Co  ^4 
Cn    to 

O^    HH 

00     M 

vO    to 

vO    O 
Cn    ON 
vO  Co 

M    ON 
Cn    00 
4=-    to 

vO  **4 

00  O 
VO  00 
Cn    00  w 

Co  Cn 
^4  Cn 

Cn    to 
^4  VO 

ONCn 
vO    to 

oooo 

•»4    ON 

*»4      M 

•^J   O 

M     ON 

O  **^J 

oo  oo 

Co     tO 
^4  Cn 

oo  oo 

vO  VO 
to  4* 

OO  00 

o  o 

vO    00 

ONCn 

00  00 
Co  Co 
to   to 

\o  ^o 

Co  Co 

00  00 

oo  oo 

vo  vO 

o  -^  *. 

0      M 

M  Cn 

ON  ON 

ON  Co 

to   oo 

M    ON 

00  O 

to  Co 
•-~4    M 

oo  o 
00  CO 

M      O 

O     00 

£3 

0    0 

to   to 

0    0 

to   to 

££ 

M      M     01 

to    ON 

Co  ^4 

to   to 

ON  to 
^4    tO 

M      tO 

00     ON 
O   «-4 

M    <«4 

tO      M 

M      0 

to  co 

vO    to 
Cn    O 
00  O     ON 

4»    00 

VO      M 

£  "2 

vO    O 
O   Cn 

•<l      M 

O  4* 

ON  4^ 
00  O 

00^4 

OOCn 

O    ON 
ON  10 

0    0 

0    0 

0    0 

0    0 

0    0 

0    0 

CO  Cn 
Cn    O 

to   to 

ON   tO 

0    0 
O^  *^4 

to    to 

**4  Co 

O    O 
4^-  On 

88 

Co  Co   ^ 
On    to 

0    0 

0    O 

O    0 

0    0 

0    O 

0    0 

0    0 

^4  Cn 

M     M 
00      M 

0    0 
Oo  4^ 

oo  4* 

0    0 
to  Co 

88 

•^4    ON   ' 

28 

Number  25  presents  a  similar  discrepancy,  although  of  small 
magnitude.  However,  the  errors  are  too  large  to  warrant 
the  use  of  the  manometer,  in  both  cases,  in  osmotic  pressure 
work,  until  their  nitrogen  volumes,  at  standard  conditions, 
have  again  been  carefully  determined. 

Any  percentage  error  in  volume,  as  expressed  in  column 
7  of  Table  II,  is  the  whole  error  of  the  two  manometers. 
Now,  since  the  two  manometers  are  connected  by  free-flow- 
ing mercury,  the  case  is  not  unlike  that  of  a  balance.  If 
one  arm  of  the  balance  moves  in  one  direction,  the  other  arm 
moves  in  the  contrary  direction.  Then,  in  the  case  of  the 
two  connected  manometers,  even  a  small  displacement  of 
equilibrium,  for  any  cause  whatsoever,  would  divide  itself 
between  the  two  instruments.  Therefore  it  is  unfair  to  make 
one  monometer  carry  the  error  of  the  two.  For  this  reason 
the  writer  believes  that  column  8  expresses  the  real  error 
more  closely  than  column  7. 

The  comparisons  of  these  manometers  will  be  carried  on 
under  pressures  as  high  as  28  to  30  atmospheres.  For  this 
work  the  "steel  block"  and  the  "brass  block"  will  be  used. 
The  brass  block  differs  from  the  steel  block  in  that  its  reser- 
voir contains  water,  and  that  manometers  may  be  placed  into 
it  in  the  same  fashion  as  they  are  placed  in  the  cells  for  an 
osmotic  pressure  measurement. 


SUMMARY. 

1.  The  earlier  work  on  manometers  has  been  reviewed, 
and  its  inaccuracies  taken  into  account. 

2.  A   new    type    of    manometer   has    been    devised.      Its 
advantages  over  the  older  type  have  been  pointed  out  and 
discussed. 

3.  The  method   of  calibration  of  these   manometers  has 
been  reviewed. 

4.  A  supply  of  very  high  grade  mercury  has  been  prepared 
and  the  method  described. 

5.  The   method   of   determining   capillary   depressions   of 
the  manometer  tubes  has  been  dealt  with  and  the  purpose 
fully  discussed. 

6.  A   review   of   the   method  for  filling   the   manometers 
with  nitrogen,  and  of  closing  the  instruments  has  been  given, 
and  the  method  of  preparation  of  the  nitrogen  gas  has  been 
described. 

7.  The  apparatus,   bath,   and  method   of   calculation  for 
determination  of  gas  volumes,   at  standard   conditions,   of 
the  manometers  has  been  described. 

8.  Comparisons  of  all  the  new  manometers  have  been  made 
against  each   other   at  low   pressures;   and    15   satisfactory 
instruments  have  been  obtained. 

9.  The  object  of  the  work  was  to  throw  light  on  error  sources 
of  manometers  of  small  bore.     Although  much  remains  yet 
to  be  done,  it  is  believed  that  some  progress  toward  the  end 
has  been  made. 


BIOGRAPHY. 

The  writer  was  born  on  a  farm,  six  miles  west  of  Brooks- 
ville,  Mississippi,  on  May  loth,  1884.  He  was  awarded  the 
B.S.  degree,  from  Mississippi  College,  Clinton,  Miss.,  in  June 
1904;  A.M.,  Ibid.,  1905.  He  has  spent  three  years  in  residence 
in  the  Johns  Hopkins  University. 


INITIAL  FINE  OF  25  CENTS 


BOOK 


OVERDUE. 


YC   I  1318 


C 


3 


